Use Tether's addTargetClasses option instead of manually removing classnames

Created by: lincolndbryant

As of Tether 0.7.1 (May 2015) - we introduced addTargetClasses, which prevents adding any classnames to the target el when false. I confirmed this with the visual tests, but can add a unit test if helpful.

I found the issue referenced in the comment and will resolve, sorry that got lost.

Thanks so much for incorporating Tether!